The present invention relates to a nitride semiconductor laser device and a method for fabricating the same, and more particularly, it relates to a self-pulsation nitride semiconductor laser device having a buried type current blocking structure and a method for fabricating the same.
Currently, a group III-V nitride-based compound semiconductor including group III elements of aluminum (Al), gallium (Ga) and indium (In) and a group V element of nitrogen (N), typified by gallium nitride (GaN) and represented by a general formula, InXGaYAl1-X-YN (wherein 0≦X≦1, 0≦Y≦1 and X+Y≦1), i.e., what is called a nitride semiconductor (hereinafter referred to as a GaN-based semiconductor), is regarded remarkable. With respect to, for example, an optical device, a light emitting diode (LED) using a nitride semiconductor is used in a large display device, a traffic light and the like. Also, a white LED obtained by combining an LED using a nitride semiconductor and a fluorescent material is partially commercialized and is expected to be substituted for currently used lighting equipment when the luminous efficiency is improved in the future.
Furthermore, a violet semiconductor laser device using a nitride semiconductor is now being earnestly studied and developed. As compared with a conventional semiconductor laser device emitting red or infrared light used for an optical disk such as a CD or a DVD, a spot diameter obtained on the optical disk can be reduced in using the violet laser device, and hence, the recording density of the optical disk can be improved.
A violet semiconductor laser device currently practically used employs a ridge structure as shown in FIG. 9. In this structure, a ridge 101 is formed by dry etching, and the lateral mode is controlled by adjusting the width and the depth of the ridge.
In this ridge structure, however, since an electrode 102 should be formed on the ridge 101, the area range of the electrode is restricted. Also, since the ridge 101 is formed by the dry etching, the depth of the ridge is varied, resulting in varying the lateral mode characteristic. Due to such problems in the structure and the fabrication, a violet semiconductor laser device with sufficient performance and reliability has not been realized in a good yield.
On the other hand, a buried type laser device as shown in FIG. 10 is employed for a GaAs-based semiconductor laser device but not yet employed for a GaN-based semiconductor laser device. This is because it is difficult to stably etch a GaN-based semiconductor with small damage. A general method for processing a GaN-based material is dry etching, and when an opening 104 is formed in a current blocking layer 103, if the dry etching is employed, a damage is caused in the vicinity, which degrades the device characteristics. Alternatively, wet etching is generally employed for etching with small damage, but wet etching technique for a GaN-based material with high reproducibility has not been established yet. In addition to such processing technique for a GaN-based semiconductor, crystal of a GaN-based semiconductor is difficult to grow, and it is difficult to regrow a cladding layer 106 with high crystallinity after forming the opening 104 in the current blocking layer 103. This is probably another reason why the buried type structure is not employed.
However, in employing the buried type structure, a distance from an InGaN active layer to the current blocking layer 103 affecting the lateral mode characteristic can be accurately controlled, and serial resistance can be reduced because a contact electrode can be formed in a large area. Thus, the buried type structure is variously advantageous to the ridge structure in the performance and the reliability.
Therefore, some techniques for overcoming the aforementioned problems of the buried type structure peculiar to a GaN-based semiconductor have been proposed.
Japanese Laid-Open Patent Publication No. 2003-78215 discloses a technique to improve the reproducibility of etching for an opening of a current blocking layer by increasing etch selectivity of a GaN-based semiconductor. Specifically, after forming an amorphous current blocking layer on a crystalline cladding layer, the current blocking layer is partly etched by wet etching using a phosphoric acid-containing solution, and thereafter, annealing is performed at a high temperature, so as to crystallize the amorphous current blocking layer.
According to this technique, in etching the amorphous current blocking layer, since an etching ratio between an amorphous layer and a crystalline layer is large, the underlying crystalline cladding layer can be used as an etching stopper, and hence, the etching of the current blocking layer can be well controlled.
However, although the amorphous layer is crystallized through the annealing at a high temperature after forming the opening in the amorphous current blocking layer, crystal of a GaN-based semiconductor is difficult to grow as described above, and a re-crystallized layer obtained through the high temperature annealing does not always have high quality. Furthermore, even when a regrown layer is formed on the current blocking layer with such low crystallinity, the regrown layer is difficult to attain high crystallinity.
Japanese Laid-Open Patent Publication No. 10-93199 discloses a technique to form an opening in a current blocking layer without etching an underlying cladding layer by forming a re-evaporation layer working as an etching stopper between the cladding layer and the current blocking layer. In this case, the re-evaporation layer exposed in the opening of the current blocking layer is made of a material that can be selectively removed through evaporation by annealing after forming the opening in the current blocking layer. This procedure for evaporating the re-evaporation layer can be performled in a MOCVD system, and hence, without exposing the exposed underlying cladding layer to the air but while keeping its surface clean, the formation of a regrown layer can be performed subsequently to the evaporation procedure. Thus, the regrown layer can be formed with high crystallinity.